GIFT  OF 
ENGINEERING  LIBRARY 


Voltage  Testing  of  Cables 


Middleton  and  Dawes 


REPRINTED  BY 


SlMPlEXfRE&CABI£(9 

MANUFACTURERS 

201  DEVONSHIRE  ST.    BOSTON 

CHICAGO          SAN  FRANCISCO 


Voltage  Testi 


By  W.  I.  Mi 
Chester  L. 


A  paper  read  \une^&,   191 
n  Institu 
ectricah  Engineers 


REPRINTED  BY 


SIMPLEX  IRE  &CABIE@ 

MANUFACTURERS 
2O1  DEVONSHIRE  ST..   BOSTON 

CHICAGO          SAN  FRANCISCO 


THE  SOUTHGATE  PRESS  —  T.  W.  RIPLEY  Co. 
BOSTON,  MASS. 


GIFT  OF 
HNGINCCf^NG  LIBRARY 


.    •'• 


Voltage  Testing  of  Cables 

We  are  pleased  to  present  the  following  reprint  of 
"Voltage  Testing  of  Cables,"  because  it  contains  three 
features  of  general  interest  to  engineers: 

(1)  A  technical  discussion  indicating  a  rational 
method  of  standardizing  voltage  tests  on  in- 
sulated conductors. 

(2)  A  consideration  of  some  difficulties  encoun- 
tered in  making  voltage  tests,  and  methods 
of  overcoming  them. 

(3)  A  description  of  an  instrument  based  on 
the  oscillograph  principle,  with  which  the 
maximum  voltage  may  be  determined  re- 
gardless of  wave  form. 

JMr.  Middleton,  as  electrical  engineer  in  charge  of  our 
Testing  Department  for  twelve  years,  has  made  a  careful 
scientific  study  of  this  subject,  in  addition  to  becoming  thor- 
oughly familiar  with  its  practical  side.  Mr.  Dawes,  an  in- 
structor at  Harvard  University  and  at  the  United  States 
Naval  Academy,  Annapolis,  has  aided  greatly  by  his  knowl- 
edge of  the  theory  and  mathematics  of  the  subject. 

The  experiments  described  in  the  paper  were  made  in 
our  factory.  The  voltmeter  has  been  developed  by  us,  owing 
to  the  lack  of  any  simple  instrument  available  for  reading 
peak  voltages. 

SIMPLEX  WIRE  &  CABLE  CO. 

Boston,  September,  1914 


SG5706 


VOLTAGE  TESTING  OF  CABLES 


BY  W.  I.  MIDDLETON  AND  CHESTER  L.  DAWES 


ABSTRACT  OF  PAPER 

In  this  country  rubber  compound,  paper,  and  cambric  are 
generally  used  for  cable  insulation.  From  the  formula 

C\  ft  f\  S   T/^ 

5  =  — — -,  the  stress  at  any  point   in  a   homogeneous  insula- 

d  logic— - 
a 

tion  may  be  determined.  The  minimum  stress  and  the  maximum 
allowable  voltage  occur  when  the  conductor  is  10/27  of  the  sheath 
diameter.  The  present  irrational  practise  of  testing  cables 
should  be  standardized  to  conform  to  this  formula  or  a  modifica- 
tion of  it. 

Over-stressing  of  the  insulation  is  accompanied  by  a  change 
of  insulation  resistance  and  electrostatic  capacity. 

No  one  factor  of  safety  is  applicable  to  every  cable  system, 
but  one  must  consider  the  conditions  of  operation  as  well. 

In  testing,  the  voltage  may  be  applied:  (1)  by  submersion;  (2) 
between  the  conductor  and  metallic  sheath;  (3)  between  wires. 
The  submersion  test  is  the  most  severe.  A  sine  wave  is  de- 
sirable for  testing  purposes,  but  rarely  occurs  in  a  commercial 
generator  under  these  severe  conditions  of  load.  Reactance 
cannot  always  be  used  successfully  to  reduce  the  volt-ampere 
load  on  the  generator. 

With  a  distorted  wave  an  a-c.  voltmeter  gives  only  a  poor 
indication  of  the  maximum  voltage.  The  writers  have  de- 
vised an  instrument  based  on  the  oscillograph  principle,  with 
which  the  maximum  voltage  may  be  determined,  regardless  of 
wave  form. 

THE  design  of  cables  is  largely  dependent  on  data  obtained 
from  voltage  tests  made  on  commercial  lengths.  Such  tests 
are  usually  conducted  in  the  testing-room  but  are  frequently 
made  after  the  cable  has  been  installed.  The  importance  of  this 
subject  has  led  the  writers  to  present  such  data  as  may  seem 
either  useful  or  of  interest  in  connection  with  the  design  or  test- 
ing of  cables,  and  further,  to  enumerate  some  of  the  difficulties 
encountered  in  making  such  voltage  tests,  together  with  the 
methods  adopted  to  eliminate  these  difficulties. 


INSULATING  MATERIALS 

In  this  country,  three  materials  are  in  general  use  for  the  in- 
sulation of  wires  and  cables;  rubber  compound,  varnished  cam- 
bric, and  paper. 


6  VOLTAGE    TESTING   OF    CABLES 

Rubber  compound  is  the  oldest,  oixcLis  the  only  one  that  can 
be  used  under  all  conditions  without  the  aid  of  a  lead  sheath. 
Its  composition  id  tttdre  cprtigle^  £ha>l{that:of  the  others,  involv- 
ing pure  rubber,  certain  mineral  ingredients,  and  hydrocarbons. 
The  number  of  such  ingredients  and  the  proportion  of  each  that 
can  be  used  has  allowed  a  great  number  of  compounds  to  be  made 
and  has  led  to  considerable  discussion  as  to  the  value  of  some  of 
these  as  insulating  materials. 

Paper  as  an  insulation  for  wires  and  cables  is  used  in  two  ways : 
wrapped  on  loosely  and  kept  dry,  as  in  telephone  cables,  or  put 
on  tightly  and  saturated  with  some  good  insulating  oil  or  com- 
pound. The  insulating  properties  of  this  class  of  cable  depend 
absolutely  on  the  soundness  of  the  lead  sheath. 

Varnished  cambric  is  the  most  recent  material  used  for  cable 
insulation  and  stands  between  rubber  and  paper;  it  has  a  number 
of  good  qualities.  Being  a  cotton  fabric  coated  on  both  sides 
with  several  films  of  insulating  varnish,  it  is  almost  water  proof, 
and  may  be  submerged  in  water  for  a  considerable  length  of  time 
without  undue  deterioration.  In  the  process  of  manufacture,  the 
varnished  cloth  is  applied  spirally  in  the  form  of  tape,  a  viscous 
insulating  compound  being  simultaneously  applied  between 
layers. 

VOLTAGE  AND  STRESS  FORMULAS 

Theoretically,  the  stress  at  any  point  on  a  homogeneous 
cylindrical  insulation  may  be  determined  from  the  following 
formula : 

5  _  0.434   V 

Xloglo—  (1) 

r 

where   V  =  volts  impressed  between  conductor  and  sheath, 
r  =  radius  of  the  conductor, 
R  =  radius  of  the  insulation, 

X  =  distance  from  the  axis  to  the  point  in  question, 
5  =  stress  in  volts  per  unit  thickness  of  insulation  at  this 

point. 

The  stress  will  be  a  maximum  at  the  surface  of  the  conductor. 
Therefore  letting  X  =  r,  r  =  d/2,  and  R  =  D/2,  the  stress  at 
the  surface  of  the  conductor  becomes 

0.434    V  0.868  V 


S  = 


d  t          D    '  '     .          D  (2) 

— log  10  — -       cnog10  — 
la  a 


VOLTAGE    TESTING  OF   CABLES 


where  d  =  diameter  of  the  conductor, 
D  =  diameter  of  the  insulation. 
This  relation  is  shown  in  Fig.  1. 
With  D  and    V  fixed,  the  maximum  stress  at  the  surface  of 

any  insulated  wire  will  be  inversely  proportional  to  d  logio   — ' 

d 

It  will  therefore  diminish  with  an  increase  in  the  diameter  of  the 
conductor,  until  a  minimum  is  reached,  after  which  the  stress 
will  increase  with  further  increase  of  conductor  diameter.  This 
minimum  may  be  found  by  differentiating  formula  (2),  and 


0  40  8O  120  160  ZOO  240         ?flQ 

CONDUCTOR  DIAMETER -Mi  15 

FIG.  1 — CURVE  OF  STRESS  AND  CONDUCTOR  DIAMETER. 

Voltage  (V)  constant  at  10,000.     Diameter  over  insulation  (D)  constant  at  272  mils. 

equating  to  zero,  and  the  value  of  d  corresponding  thereto  is 
found  to  be  £>/€=  D/2.72  where  e  is  the  Napierian  base.  This 
relation  plotted  with  volts  per  mil  as  ordinates  and  conductor  di- 
ameter as  abscissas,  is  shown  in  Fig.  1.  Point  A  shows  the 
point  of  minimum  stress.  The  wire  diameter  for  minimum 
stress  is  about  10/27  of  the  diameter  of  the  insulation. 

If  in  formula  (2),  D  and  the  maximum  allowable  stress  5  are 
kept  constant,  and  the  voltage  is  allowed  to  vary  with  the  con- 
ductor diameter,  we  have 


V 


Sd 

- 
0.868 


D 

— 

d 


(3) 


8 


VOLTAGE    TESTING   OF   CABLES 


This  relation  is  shown  in  Fig.  2.  Under  these  conditions  the 
maximum  voltage  that  we  may  impress  between  the  conductor 
and  the  outside,  without  exceeding  the  allowable  stress,  occurs 
whend  =£>/2.72. 

This  does  not  mean,  however,  that  if  this  maximum  voltage 
were  impressed  upon  the  cable  when  d  is  less  than  D/2.72  the 
insulation  would  break  down,  but  rather  that  the  wall  of  insula- 
tion between  the  diameter  D/2.72  and  the  conductor  would  be 


2QPO 


ISO          200         250  300          350 

CONDUCTOR   DIAMtTER-MILS 

FIG.  2 — RELATION  BETWEEN  TEST  PRESSURE  AND  CONDUCTOR  DIAMETER 
E  =  K  d  log  D  /  d.     K  =  200.     D  =  544  mils.     Stress  constant. 

All  wires  having  the  same  outside  diameter  whose  conductor  diameter  is  equal  to  or  less 
than  D/2.72  (=  dc)  should  have  the  same  breakdown  voltage. 


stressed  beyond  the  allowable  limit.  The  layer  nearest  the  con- 
ductor is  under  the  maximum  stress,  and  the  stress  in  any  other 
layer  is  inversely  proportional  to  its  distance  from  the  center  if 
the  electrical  characteristics  of  the  insulation  remain  unchanged. 
Theoretically,  then,  all  cables  having  d  less  than  D/2.72  should 
break  down  at  the  same  voltage,  hence  follow  the  line  ABC,  if 
it  be  assumed  that  the  voltage  drop  across  the  over-stressed 
layer  is  practically  zero. 

Although  there  has  been  no  evidence,  so  far  as  the  writers 


VOLTAGE    TESTING   OF   CABLES 


9 


know,  that  these  inside  layers  are  actually  broken  down  under 
these  conditions,  it  is  a  well-known  fact  that  the  dielectric  con- 
stant of  an  over-stressed  dielectric  is  greater  than  the  normal 
constant  before  breakdown,  and  this  tends  to  reduce  the  voltage 
drop  across  the  inner  layers  and  throw  more  stress  on  the  outer 
wall.  Whether  or  not  this  be  true,  experience  indicates  that  the 

TABLE  I 

Values  of  d  Iog10  —  .     D  and  d  in  mils 


Size  Wire  B.  &  S. 

Wall 
(in.) 

No.  14 

No.  12 

No.  10 

No.  8 

No.  6 

No.  4 

No.  1 
Std. 

4/0   Std. 

1.000.000 
Cir.  Mfls 

1/32 
3/64 
2/32 

19.1 
25.2 
30.3 

20.0 
27.1 
33.0 

21.3 
28.9 
36.4 

21.9 
30.7 
38.0 

22.8 
33.0 
40.5 

23.9 
33.7 
42.4 

24.8 
36.0 
46.0 

25.6 
37.6 
49.4 

26.2 
38.8 
51.6 

5/64 
3/32 

7/64 

34.5 
38.4 
41.6 

38.0 
42.2 
46.0 

41.1 
46.3 
50.8 

44.2 

50.4 
55.6 

47.6 
54.4 
60.0 

50.4 
58.0 
65.1 

55.5 
64.5 
73.0 

59.5 
70.0 
79.6 

64.0 
75.5 
87.1 

4/32 
9/64 
5/32 

44.6 
47.2 
49.7 

49.5 
52.7 
55.5 

54.9 

58.6 
62.2 

60.4 
64.8 
69.0 

65.9 
70.9 
75.6 

71.0 
76.9 
82.5 

80.7 
88.1 
95.5 

89.0 
97.6 
107.0 

98.4 
108.9 
119.2 

6/32 

7/32 
8/32 

54.0 
57.7 
61.0 

60.8 
65.3 
69.3 

68.3 
73.8 
78.6 

76.3 
82.9 
88.8 

84.5 
92.2 
99.1 

92.5 
102.0 
110.0 

108.5 
121.0 
132.0 

123.2 
138.9 
153.0 

141.1 
161.2 
180.4 

9/32 
10/32 

64.0 
66.3 

72.9 
76.2 

83.0 
87.0 

94.1 

98.9 

105.3 
111.3 

117.3 
124.5 

141.0 
150.5 

166.2 
179.3 

198.8 
217.0 

breakdown  occurs  along  the  line  A  By  Fig.  2,  and  little  or  nothing 
is  gained  in  making  d  less  than  D/2.72. 

The  following  formula  has  therefore  been  adopted  as  most 
nearly  representing  the  breakdown  stress  for  small  conductors 
with  a  heavy  wall  of  insulation.* 


0.868  V 
D 


(4) 


where  cfc  =  D/2.72. 


'Potential  Stresses  in  Dielectrics,  by  H.  S.  Osborne,  TRANS.  A.  I.  E.  E., 
Vol.  XXIX,  part  2,  p.  1553. 

Discussions,  by  W.  I.  Middleton,  p.  1587;  Henry  A.  Morss,  p.  1589;  Wm. 
A.  Del  Mar,  p.  1614. 


10 


VOLTAGE    TESTING   OF    CABLES 


The  maximum  voltage  that  may  be  safely  impressed  upon  a 

- 

d 


cable  of  a  given  insulating  material  is  proportional  to 


and  to  a  constant  K,  depending  on  the  quality  of  material 
(formula  3). 

Values  of  d  logio  —  are  given  in  Table  I,  for  walls  from  1/32 
d 

in.  (0.794  mm.)  to  10/32  in.  (7.94mm.)  thick  on  various  wires 
from  No.  14  B.  &  S.  to  1,000,000  cir.  mil  cable,  and  these  values 
are  plotted  in  Fig.  3.  These  tables  and  curves  are  very  useful 


WALL  OF  INSULATION,  64 r-h  i N.(OJ97mm.) 

FIG.  3 — RELATION  BETWEEN  d  LOGIC  D/d  AND  WALL  OF  INSULATION. 

"d"    AND    "D"    IN    MILS 

in  the  application  of  the  formula  to  cable  testing,  for  the  value 
of  K  only  needs  to  be  known  to  determine  the  allowable  voltage 
test  or  proper  diameter.  When  d  is  expressed  in  mils,  K  varies 
from  100  to  250  for  rubber  compounds,  is  about  250  for  cambric, 
and  for  paper  with  thin  walls  is  much  less  than  250  but  is  greater 
than  this  for  walls  exceeding  10/32  in.  (7.95  mm.) 

STANDARDIZATION  OF  VOLTAGE  TESTS 

Until  recently  no  attempt  has  been  made  to  standardize  volt- 
age tests  on  insulated  wires  and  cables  with  reference  to  the  theo- 
retical stress.  The  result  is  a  chaotic  condition  of  affairs.  A  very 


VOLTAGE   TESTING  OF   CABLES  11 

common  rule  has  been  to  specify  a  test  of  2J/£  times  the  working 
pressure,  the  feeling  being  that  this  allows  a  good  factor  of 
safety.  This  rule  might  be  satisfactory  if  the  wires  were  all  of 
one  size  and  the  same  wall  of  insulation  used  for  each  working 
pressure.  As  such  is  not  the  case,  the  only  rational  way  to  test 

all  cables  is  by  the  d  logio  —  rule  or  a  modification  of  it. 

d 

On  October  1,  1905,  the  Underwriters'  Laboratories  specified 
that  all  Code  wires  for  voltages  between  0  and  600  volts  should  be 
tested  after  ten  hours  immersion  in  water,  with  1500  volts  (alter- 
nating current)  for  not  less  than  five  seconds.  That  specifica- 
tion showed  the  influence  exerted  on  most  engineers  at  that 
time  by  the  factor  of  2J/£  times  the  working  pressure. 

A  little  study  of  Fig.  3  together  with  the  sizes  of  wires  and  walls 
of  insulation  made  under  the  code  specifications  shows  how 
absurd  this  test  was.  The  conductors  varied  from  No.  14  to 
1,000,000  cir.  mils  and  larger  (0.064  to  1.156  in.  or  1.63 
to  29.4  mm.)  in  diameter;  the  wall  of  insulation  from  3/64  in. 
(1.19  mm.)  to  7/64  in.  (2.78  mm.),  (0.0469  to  0.109  in.).  If  the 
3/64  in.  (1.19  mm.)  wall  of  insulation  on  the  No.  14  would  stand 
1500  volts,  it  should  surely  stand  much  more  than  this  on  the 
1,000,000  cir.  mil.  It  would  therefore  be  possible  for  the 
1,000,000  cir.  mil  cable  to  meet  this  test  even  were  the  wall  of 
insulation  defective  or  actually  less  than  3/64  in.  (1.19  mm.) 
in  places,  whereas  the  main  object  of  the  voltage  test  is  to  break 
down  any  such  faults. 

The  1911  Code  specifications  for  0  to  600  volts  have,  in  part, 
remedied  this  defect  by  calling  for  a  test  of  the  1,000,000  cir.  mil 
at  3500  volts,  but  at  the  higher  voltages  they  still  hold  to  2% 
times  the  working  pressure.  The  specifications  of  some  of  the 
largest  buyers  in  the  country  to-day  are  equally  inconsistent. 

This  is  a  lamentable  condition.  It  allows  too  much  variation 
in  the  dielectric  strength  of  the  insulating  materials.  Some  of 
this  variation  may  be  due  to  ignorance,  and  some  may  be  in- 
tentional. When  a  cable  is  tested  at  only  one-half  the  voltage 
to  which  it  should  be  subjected,  there  results  in  many  instances 
a  carelessness  in  its  manufacture.  The  writers  believe  that  too 
little  has  thus  far  been  accomplished  in  the  line  of  the  standardi- 
zation of  cable  testing,  when  compared  with  other  branches  of 
engineering. 

The  following  tests,  in  Tables  II  and  III,  are  recommended 


12 


VOLTAGE    TESTING   OF    CABLES 


a 
3 


S  8  2 


ete}Okeot* 


10  Tfi  eo  ec  c<i  I-H  o 


(N     rH     rH     O    OS     00 


~*  o  O5  O5  oo 


Bs-*5* 

*s 


e  = 

u     u 

"o  *u 


VOLTAGE    TESTING   OF    CABLES 


13 


for  high-and  medium- voltage  cables,  respectively.  Table  IV,  on 
the  other  hand,  is  the  specification  as  called  for  by  a  purchaser  for 
the  cables  listed  in  Table  III.  It  will  be  seen  that  the  testing 
pressure  recommended  is  from  !}/£  to  2J/2  times  that  called  for 
by  the  purchaser.  As  far  as  working  pressure  is  concerned,  the 
factor  of  safety  demanded  by  the  purchaser  in  Table  IV  is,  with- 
out doubt,  sufficient.  Yet  the  tests  called  for  by  this  table  would 
not  begin  to  show  up  any  but  the  most  serious  defects  in  the  in- 
sulation of  the  respective  cables. 

The  *A.  R.  E.  E.  committee  on  "Wire  and  Cable  Specifica- 
tions" has  taken  the  most  important  step  thus  far  in  the  stand- 
ardization of  voltage  tests  for  cables,  in  its  recent  recommenda- 
tions for  tests  on  rubber,  cambric  and  paper  insulation. 

From  Fig.  3  it  will  be  noted 
that  there  is  but  a  relatively 
small  difference  in  the  values  of 


^ 

^ 

^if 

,••• 

^ 

^ 

^ 

^^ 

n 

. 

x^ 

X1 

^"^ 

^ 

X 

/^ 

' 

ty/ 

^ 

0 

5 

10            1. 

20 

25 

it 

for   diameters  corre- 


d   logic - 

a 

spending  to  No.  1  and  No.  4/0 
B.  &  S.  gage,  consequently  one 
set  of  values  covering  these 
ranges  has  been  recommended, 
and  is  plotted  in  Fig.  4. 

Curve  I  is  recommended  for 

IDUCTORS?TRECOMLMENDED  B?  A.°R~  PaPer    and   varnished    cambric, 
E.  E.    No.  l  TO  No.  4/0  A.  W.  G.  curve  II  for  rubber,  and  curve 


I. 

II. 

III. 


Paper  or  varnished  cambric. 

Rubber. 

155  d  logio  D/d  for  No.  1  A.  W.  G. 


HI    fa    the    yalue   Qf     Kd   fog 

, 

when    K  =  155   and  d  and  D 

are  given  in  mils.  Up  to  a  20/64-in.  (7.95-mm.)  wall  the  paper 
and  cambric  are  rated  at  a  lower  working  pressure  than  the 
rubber,  for  mechanical  reasons,  but  above  this  they  should  test 
even  better  than  the  rubber. 

OVER-STRESSING  CABLES 

Much  has  been  said  in  the  past  relative  to  unduly  severe 
testing  conditions  in  that  the  insulation,  initially  sound  mechani- 
cally, becomes  stressed  beyond  the  electric  elastic  limit  when 
tested.  Although  the  short  duration  of  the  test  may  not  develop 
any  faults,  the  cable  is  nevertheless  permanently  injured,  hence 
less  able  to  withstand  the  shocks  incidental  to  service  conditions. 

*Association  of  Railway  Electrical  Engineers. 


14 


VOLTAGE   TESTING  OF   CABLES 


TABLE    III 

RECOMMENDED  VOLTAGE  TESTS  FOR  LOW-VOLTAGE  CABLES. 

INSULATED  WITH  LOW-TENSION  RUBBER  COMPOUND.     VOLTAGE  TEST  AT  FACTORY  FIVB 
MINUTES  AS  PER  TABLE;  VOLTAGE  TEST  AFTER   INSTALLATION  30  MINUTES  AT  50  PERCENT 

OF  TABLE  VALUE. 


Size  conductor 

Minimum  thickness  insula- 
tion, inch. 

Test  pressure,  volts. 

Stranded 

...."  1,000,000  cir.  mils 

4/32 

10,000 

750,000 

4/32 

10,000 

500,000 

4/32 

9,000 

S50.000 

4/32 

9,000             i'P 

4/0  A.  W.  G. 

3/32 

7,000 

2/0 

3/32 

6,500 

1/0        " 

3/32 

6,500 

2 

3/32 

6,000 

Solid 

4  A.  W.  G,       . 

3/32 

5,500 

6 

3/32 

5,500 

8 

3/32 

5,000 

10 

3/32 

4,500 

12 

3/32 

4.000 

14 

5/64 

3,500 

14  (3)  "  conductor 

5/64 

3,500 

TABLE   IV 
VOLTAGE  TESTS  AS  SPECIFIED  BY  A  PURCHASER. 

TESTS  ON  CABLES   INSULATED   WITH   LOW-TENSION   RUBBER  COMPOUND.     VOLTAGE   TEST  AT 
FACTORY  FIVE  MINUTES  AS  PER  TABLE;  VOLTAGE  TEST  AFTER  INSTALLATION  30  MINUTES  AT 
80  PERCENT  OF  TABLE  VALUE  ,'  •  ••  > 


Size  conductor 

Strands 

Wall. 

Volts  work- 
ing pressure 

Volts  test 
pressure 

r.     .Stranded 

1,000,000  cir.  mils 

61 

4/32 

1000 

4000 

750,000      > 

61 

4/32 

•• 

4000 

500,000 

37 

4/32 

" 

4000 

350,000 

37 

4/32 

" 

3000 

4/0  A.  W.  G. 

19 

3/32 

•' 

3000 

2/0 

19 

3/32 

" 

3000 

1/0 

19 

3/32 

" 

3000 

2 

7 

3/32 

.,                   ** 

3000 

Solid 

4  A.  W.  G. 

3/32 

1000 

3000 

6 

3/32 

" 

3000 

8 

3/32 

" 

3000 

10 

3/32 

" 

3000 

12 

3/32 

" 

3000 

14 

5/64 

" 

2000 

14 

3-Cond. 

5/64  (no  belt) 

2000 

VOLTAGE    TESTING  OF   CABLES 


15 


This  may  be  the  case,  but  fortunately  the  insulation  resistance 
and  the  electrostatic  capacity  enable  us  to  determine  the  degree 
to  which  the  insulation  has  been  over-stressed. 

Immediately  after  the  stress  is  applied,  the  insulation  resistr 
ance,  measured  with  direct  current,  may  drop  considerably 
below  its  initial  value  as  obtained  a  few  moments  previous  to  the 
application  of  voltage.  This  change  may  be  as  great  as  50  per- 
cent. If  further  readings  of  insulation  resistance  are  taken,  they 


TABLE  v 

WIRES  SHOWING  RESULTS  OF  STRESS. 
Megohms  in  1000  ft. 


Test 

Feet 

Before 

2500 

5000 

After  2 

5000 

After  2 

No. 

voltage 

volts 

volts 

hours 

volts 

hours 

1  min. 

1  min. 

5  min. 

1 

1562 

14,500 

14,500 

7,500 

11,500 

2 

1547 

22,000 

22,000 

16,000 

18.000 

3 

3150 

7.500 

7,500 

6.000 

7.000 

5,000 

5,000 

4 

1740 

15,000 

15,000 

6.500 

10.000 

750 

2.500 

5 

2402 

15,000 

15,000 

7,500 

10.000 

2,500 

3.500 

Megohms  in 

1000  ft.  after 

Break- 

repair.    4000 

down 

volts,  one 

voltage 

min. 

6 

3560 

4,800 

4,620 

13.000 

4,400 

7 

1425 

3.500 

3,440 

12.000 

4,470 

8 

2350 

9.000 

9,015 

15.000 

8,425 

9 

2750 

7,660 

7.660 

15,000 

9,150 

10 

2400 

2,950 

2,740 

7,500 

2.810 

will  show  a  gradual  increase  and  will  approach  their  initial  value 
if  the  cable  has  not  been  over-stressed,  whereas,  if  it  has  been 
over-stressed,  the  resistance  recovers  but  little.  Care  must  be 
taken  to  keep  the  temperature  constant  during  these  tests,  for 
insulation  has  a  very  large  resistance  temperature  coefficient. 
Table  V  shows  some  typical  data  taken  from  tests  made  on  long 
lengths  of  wire  as  they  were  passing  through  the  testing  room. 

The  insulation  resistances  in  tests  (1)  and  (2)  were  affected 
considerably  after  the  5000- volt  one-minute  test,  but  practically 
recovered  after  two  hours.  It  is  possible  that  thev  would  have 


16 


VOLTAGE    TESTING   OF    CABLES 


completely  regained  their  initial  resistance  if  allowed  sufficient 
time. 

Tests  (3),  (4)  and  (5)  were  first  made  under  the  same  conditions 
and,  except  in  the  case  of  (3),  the  recovery  was  much  poorer  than 
in  the  former  cases.  They  were  then  subjected  to  5000  volts  for 
five  minutes,  with  a  noticeable  reduction  in  the  resistances  of  (3) 
and  a  very  large  and  permanent  reduction  in  that  of  (4)  and  (5). 
These  last  two  were  permanently  injured. 

In  tests  (6)  to  (10)  inclusive,  there  was  no  marked  decrease  in 
resistance  after  the  5000-volt  test,  so  they  were  broken  down, 
repaired  by  patching  the  faults,  tested  at  4000  volts  for  one 
minute,  and  the  insulation  resistance  measured  again,  with  the 
results  shown  in  the  table.  Tests  (7)  and  (9)  gave  even  better 


TABLE   VI 

WIRES  SHOWING  THE  RESULTS  OF  STRESS. 
Microfarads  per  1000  ft. 


Feet 

Before  voltage 

After  5000  volts  for 

test 

1  min. 

3150 

0.126 

0.130 

2176 

0.146 

0.150 

2470 

0.130 

0.134 

2925 

0.130 

0.133 

2775 

0.120 

0.124 

results  than  the  initial  resistances,  due  no  doubt  to  patching  a 
localized  fault. 

Unfortunately,the  electrostatic  capacity  of  these  cables  was  not 
measured  after  every  application  of  voltage,  but  Table  VI  shows 
in  a  general  way  the  increase  of  capacity  due  to  stress  in  the 
dielectric.  For  a  given  stress  the  change  in  capacity  is  much 
smaller  than  the  change  in  resistance. 

These  results  show  that  it  is  possible  to  make  a  rubber  com- 
pound which  is  not  easily  stressed  beyond  the  electric  elastic 
limit,  and  further,  that  if  a  compound  is  so  stressed  it  is  possible 
by  means  of  the  insulation  resistance  to  determine  if  the  test  has 
been  too  severe. 

FACTOR  OF  SAFETY 

It  is  not  the  intention  of  the  writers  to  tell  the  operating  engi- 
neers what  should  be  the  factor  of  safety  in  a  cable  system.  Great 
fear  has  been  expressed  now  and  then,  that  engineers,  knowing 


VOLTAGE    TESTING  OF   CABLES  17 

that  cables  will  stand  these  high-voltage  tests,  will  be  tempted  to 
use  them  on  higher  working  pressures  than  they  should.  In 
this  connection  it  is  well  to  bear  in  mind  that  a  factor  of  2J/£ 
times  the  working  pressure  is  not  applicable  to  all  conditions. 

(1)  In  two  systems  of  the  same  kilowatt  capacity  the  cables  on 
that  system  having  the  lower  voltage  should  have  the  greater  factor 
of  safety.     This  is  because  the  surge  voltage  on  the  lower- volt  age 
system  will  be  greater  because  of  the  greater  current,and  the  maxi- 
mum possible  rise  in  voltage  is  e  =  i  \ — >  where  i  is  the  amperes 

c/ 

current  transient,  and  L  and  C  the  system  inductance  and  capac- 
ity, expressed  in  henry s  and  farads  respectively. 

(2)  In  two  systems  having  the  same  voltage,  those  cables  operating 
on  the  system  having  the  greater  kilowatt  capacity  should  have  the 
greater  factor  of  safety.     The  reason  for  this  is  obvious.     As  has 
been  frequently  observed,  transients  that  are  practically  un- 
important in  a  small  system  become  dangerous  if  allowed  to  take 
place  in  a  large  system.     The  writers  have  in  mind  a  case  where 
2300-volt  distributing  cables,  when  connected  to  a  relatively 
small  plant,  gave  practically  no  trouble,  but  later,  when  this 
smaller  system  received  its  energy  from  a  large  transmission  net- 
work, these  same  cables,  though  normally  operating  at  the  same 
voltage  as  before,  gave  so  much  trouble  that  they  had  to  be  re- 
placed by  cables  better  suited  to  the  conditions. 

METHOD  OF  TESTING 

The  voltage  test  can  be  applied  to  wires  and  cables  in  several 
different  ways;  by  submerging  the  cables  in  water;  testing  them 
against  a  metallic  covering  on  the  outside  such  as  a  lead  sheath  or 
tin  foil ;  and  testing  one  wire  against  another  when  there  is  more 
than  one  wire  in  the  cable.  The  submersion  test  is  the  most 
severe  as  the  water  makes  very  close  contact  with  the  outside 
of  the  cable  regardless  of  any  surface  irregularities  that  may  be 
present.  The  water  also  has  a  tendency  to  penetrate  into  any 
foreign  substance  that  may  be  in  the  insulating  material,  provided 
this  substance  has  any  affinity  for  water. 

All  of  these  tests  may  be,  and  generally  are,  made  on  rubber  in- 
sulated cables.  The  cambric  cables  to  be  braided  are  generally 
submerged  before  and  after  braiding;  cambric  cables  to  be  lead- 
covered  are  not  submerged,  as  considerable  trouble  in  drying 
them  is  experienced,  and  as  they  are  to  be  tested  after  the  lead 


18  VOLTAGE   TESTING   OF   CABLES 

Covering  has  been  applied,  the  submersion  test  is  not  necessary. 
Paper  cables  are  not  submerged,  and  all  tests  are  made  after 
leading. 

The  voltage  test,  as  applied  to  cables,  is  practically  the  same 
whether  it  is  made  submerged,  against  the  lead,  or  against  the 
contiguous  wires,  the  object  being  to  break  down  any  weakness 
that  may  exist  in  the  insulation.  How  much  pressure,  and  for 
how  long  it  shall  be  applied,  are  questions  that  have  long  been 
the  subject  of  much  discussion. 

For  several  reasons,  it  is  necessary  to  apply  the^  voltage  test 
to  the  finished  cable  and  not  to  a  short  sample.  (1)  It  is  desir- 
able to  break  down  any  weak  places  that  may  occur  in  the  cable, 
it  being  quite  impossible  to  avoid  entirely  such  places  in  manu- 
facture; (2)  to  satisfy  inspectors  and  purchasers  that  the  cable 
meets  specifications  as  regards  dielectric  strength;  (3).  to  obtain 
data  and  information  as  to  the  dielectric  strength  of  the  material; 
(4)  constants  obtained  in  laboratories  from  tests  of  short  lengths 
are  not  applicable  to  commercial  lengths  and  are  usually  mis- 
leading. 

TESTING  APPARATUS* 

Recommendations  have  appeared  at  different  times  regarding 
the  type  of  generator  and  transformer  that  it  is  advisable  to  use 
for  testing  purposes,  and  the  consensus  of  opinion  seems  to  be 
that  a  smooth-core  generator  with  field  control,  and  a  variable- 
ratio  transformer,  are  most  satisfactory.  As  will  be  shown  later, 
it  is  doubtful  if  the  generator  of  ordinary  design  can  maintain 
its  wave  form  under  the  severe  conditions  imposed  by  cable 
testing. 

Where  cables  of  some  length  are  to  be  tested,  a  frequency 
of  25  cycles  is  preferable  to  one  of  60  cycles,  for  the  necessary 
generator  and  transformer  capacities  are  practically  proportional 
to  the  frequency,  and  according  to  the  best  information  the 
writers  can  obtain  there  is  no  appreciable  difference  in  severity 
of  cable  tests  whether  made  at  25  or  60  cycles. 

In  the  following  tests,  made  in  the  testing  laboratory  of  a  wire 
manufacturer,  the  generator  used  was  a  motor-driven  25-kv-a., 
220-volt,  four-pole,  25-cycle,  single-phase  alternator,  having  10 
slots  per  pole,  and  a  conductor  belt  %  the  pole  pitch.  The  trans- 
former capacity  was  50  kv-a.,  220-50,000  volts.  The  secondary 
consisted  of  four  separate  12, 500- volt  coils,  capable  of  being  con- 

*  High-Tension  Testing  of  Insulating  Materials,  A.  B.  Hendricks,  TRANS. 
A.  I.  E.  E.,  Vol.  XXX,  part  I,  page  167. 


VOLTAGE    TESTING   OF    CABLES 


19 


nected  either  in  parallel,  in  series-parallel,  or  in  series.  The  high- 
tension  winding  had  a  total  of  12,512  turns,  and  the  low-tension 
55  turns.  The  reactance  voltage  was  about  6  percent. 

Fig.  5  shows  the  generator  voltage  on  open  circuit,  and  ex- 
cept for  the  tooth  harmonics,   the  e.m.f .    wave  is  practically 


FIG.  5 


FIG.  6 


sinusoidal.  For  testing  purposes,  this  wave  is  perfectly  satis- 
factory if  it  could  be  maintained  under  all  load  conditions.  In 
Fig.  6  is  shown  the  generator  voltage  wave  taken  at  220  volts 
when  the  transformer  is  connected,  and  also  the  exciting  current 
wave  of  the  transformer. 

This  transformer  exciting  current  is  about  17  percent  of  the 
rated  load  current  of  the  transformer,  but  is  34  percent  of  the 
rated  load  current  of  the  generator.  This  is  rather  high,  and 
further,  Fig.  6  and  other  experiments  showed  that  the  transfer- 


20  VOLTAGE   TESTING  OF   CABLES 

mer  iron  was  being  operated  at  unusually  high  saturation.  It 
might  well  be  argued  that  a  large  transformer  magnetizing  cur- 
rent is  desirable,  as  it  tends  to  offset  the  leading  component  of 
cable  charging  current,  but  it  should  be  remembered  that  beyond 
a  certain  core  density  the  additional  exciting  current  is  made  up 
almost  entirely  of  harmonics  which  do  not  neutralize  the  funda- 
mental. Experience  has  shown  this  to  be  undesirable  for  other 
reasons.  Examination  of  the  current  wave  in  Fig.  6  shows  that 
the  transformer  takes  a  pronounced  third  harmonic  current,  and 
this  current  reacting  on  the  generator  flux  tends  to  start  wave 
distortion,  producing  a  third  harmonic  in  the  e.m.f.  wave  as 
shown.  If  a  cable,  like  most  other  electrical  apparatus,  took  a 


FIG.  7 


comparatively  small  charging  current,  most  of  the  following 
difficulties,  due  to  wave  distortion,  would  disappear. 

Figs.  7,  8  and  9  show  various  voltage  waves  actually  obtained 
under  different  conditions  of  test.  The  voltages  on  the  cables, 
given  in  connection  with  all  the  following  oscillograms  are  ac- 
cording to  the  ratio  of  transformation,  hence,  are  not  strictly 
correct.  In  actual  practise  this  voltage  is  determined  directly 
from  the  high  tension  side  by  means  of  a  potential  transformer. 

The  reasons  for  this  distortion  are  obvious.  The  generator 
may  have,  inherently,  a  sine  wave  voltage,  but  the  transformer 
exciting  current  has  a  prominent  third  harmonic.  This  current 
and  the  single-phase  pulsating  armature  reaction  produced  on  the 
flux  wave,  will  usually  introduce  harmonics  in  the  voltage  wave  as 
shown  in  Fig.  6.  This  wave  is  communicated  to  the  transformer 
secondary  where  the  cable  intensifies  it  in  its  charging  current, 
and  it  is  reflected  back  in  the  generator  current,  and  increased 


VOLTAGE    TESTING   OF    CABLES 


21 


wave  distortion  results.  These  reactions  are  cumulative  and 
will  continue  to  increase  until  counter-reactions,  set  up  in  the 
magnetic  and  electric  circuits  become  sufficiently  great  to  balance 
them.  Generator  saturation,  generator  and  transformer  series 
leakage  reactance  and  the  phase  relations  of  the  harmonics  may 
tend  to  counteract  distortion.  The  value  of  series  reactance  may 


FIG.  8 


FIG.  9 


be  such  as  to  produce  resonance  for  one  harmonic  and  not  for 
the  others. 

The  generator  may  be  represented  by  a  coil  that  is  a  source 
of  voltage  and  having  reactance  and  resistance;  the  transformer 
may  be  replaced  by  a  shunt  impedance  of  a  value  equal  to  the 
open-circuit  impedance  of  the  transformer,  and  by  two  series 
impedances,  one  representing  the  equivalent  resistance  and  leak- 


22 


VOLTAGE    TESTING   OF    CABLES 


To  Current 
Vibrator1** 


To  Vottaqe 
Vibrator 


Alternator 

FIG.  10 — DIAGRAM  OF  CONNECTIONS 


age  reactance  of  the  primary  side,  and  the  other  the  impedance 
of  the  secondary  reduced  to  the  primary  side ;  the  cable  represents 
a  condenser  referred  to  the  primary  side  and  having  a  very  high 
effective  resistance.  Fig.  11  shows  this  condition. 

The  shunt  impedance  b  d  has  a  decided  effect  on  the  generator 
wave  form,  in  that  its  current  wave,  especially  at  the  higher  volt- 
ages, is  mostly  made  up  of  harmonics,  and  these,  reacting  on  the 


------  Trcynsfofmer  ——»•»•  -»--v* 

Series  Impedance 


FIG.  11 — EQUIVALENT  CIRCUIT  DIAGRAM 


generator  flux,  are  responsible  for  the  initial  distortion  to  which 
reference  has  been  already  made.  The  series  circuit  abode, 
consisting  of  the  transformer  primary  and  secondary  resistances 
and  leakage  reactances  and  the  condenser,  all  in  series  with  the 
generator  armature,  exerts  some  influence  on  the  generator  wave 
form.  The  magnitude  and  shape  of  the  generator  current  wave, 
may  be  dependent  on  the  relation  of  inductance  to  capacity  in 
this  circuit.  Therefore,  harmonics  may  be  intensified  or  di- 
minished depending  on  their  frequency,  on  the  transformer 


VOLTAGE   TESTING  OF   CABLES 


FIG.  12 

inductance  and  the  cable  capacity.     These  effects  are  illustrated 
in  Figs.  12  to  15  inclusive. 

The  oscillograms  shown  in  Figs.  12  and  13  were  taken  when  a 
cable  having  a  capacity  of  0.1  microfarad  was  connected  to  the 
transformer  secondary,  and  those  shown  in  Figs.  14  and  15  were 
taken  with  another  cable  having  a  capacity  of  0.13  microfarad, 
connected  to  the  transformer  secondary.  Two  tests  were  made 
with  each  cable ;  one  in  which  all  four  of  the  transformer  second- 
ary coils  were  connected  is  series  (50,000-volt)  connection,  the 
other  in  which  the  series-parallel  (25, 000- volt)  connection  was 
used.  In  order  to  compensate  for  the  change  in  transformer 


FIG.  13 


24  VOLTAGE    TESTING   OF   CABLES 

ratio  so  that  the  effective  voltage  on  the  cable  should  remain 
approximately  unchanged,  the  generator  voltage  was  practically 
doubled  when  the  change  was  made  from  the  50,000-volt  to  the 
25, 000- volt  connection. 

Figs.  12  and  14  show  the  results  obtained  with  the  50,000-volt 
connection,  and  in  each  case  the  e.m.f .  wave  is  very  nearly  sin- 
usoidal, and  no  appreciable  harmonics  above  the  third,  appear  in 
the  current  wave.  In  each  of  these  cases  the  generator  was  opera- 
ted at  low  saturation,  a  condition  in  which  it  would  be  less  able 
successfully  to  oppose  severe  reactions  on  the  flux  wave.  The 
natural  frequency  of  the  transformer  and  cable  circuit  in  the  case 


FIG.  14 


of  Fig.  12  is  110  cycles,  and  of  Fig.  14,  96.6  cycles.  The  shunt 
circuit  b  d,  owing  to  the  variable  nature  of  its  inductance,  would 
be  difficult  to  take  into  consideration,  except  in  a  very  general 
way  and  was  consequently  neglected  in  this  frequency  deter- 
mination. 

In  Figs.  13  and  15,  the  approximate  sinusoidal  e.m.f.  waves 
shown  in  Figs.  12  and  14  are  distorted,  containing  fair-size  fifth 
harmonics,  and  third  harmonics  about  50  percent  of  the  funda- 
mental. In  each  case  the  current  wave  is  even  more  distorted 
than  the  e.m.f.  wave  showing  a  seventh  harmonic,  a  fifth  equal 
in  magnitude  to  the  fundamental  and  a  third  about  twice  as 
great  as  the  fundamental.  In  these  cases,  the  natural  frequency 
of  the  circuit  was  215  and  192  cycles  respectively. 


VOLTAGE    TESTING   OF   CABLES  25 

Thus  in  each  case,  with  the  same  generator,  the  same  trans- 
former, the  same  frequency  and  the  same  cable,  an  approximate 
sine  wave  is  converted  into  a  complex  wave  by  simply  changing 
the  transformer  ratio  and  the  generator  voltage.  These  phenom- 
ena cannot  be  explained  on  the  basis  of  a  resonant  series  circuit 
for  the  best  wave  shapes  were  obtained  when  the  circuit  constants 
were  more  conducive  to  the  flow  of  the  troublesome  third  and 
fifth  harmonics.  However,  in  these  two  latter  cases  the  trans- 
former exciting  current  contains  harmonics  of  very  appreciable 
magnitude  as  shown  in  Fig.  6,  and  in  the  writers'  opinion,  these 


FIG.  15 


harmonics  are  practically  responsible  for  the  results  that  were 
obtained. 

It  might  also  be  added,  referring  to  Fig.  11,  that  in  the  circuit 
abed,  the  resistance  is  very  small,  thus  offering  excellent  op- 
portunities for  oscillations  to  take  place  during  the  transient  or 
building  up  condition.  Great  care  must  be  exercised  in  raising 
and  lowering  the  voltage,  as  the  possiblity  of  building  up  an 
abnormal  potential  across  the  cable  is  always  present,  and  this 
may  result  in  a  puncture. 

Specific  illustrations  of  this  have  come  to  the  writers'  attention 
on  several  occasions.  When  the  voltage  across  a  cable  was  being 
gradually  raised,  the  spark-gap,  connected  in  parallel,  would  dis- 
charge light  sparks,  momentarily,  when  the  switchboard  volt- 
meter indicated  that  the  potential  across  the  gap  was  only  half 
that  at  which  the  gap  was  set.  If  the  voltage  was  held  constant 


26  VOLTAGE  TESTING  OF  CABLES 

for  an  instant,  the  sparking  would  discontinue,  and  the  voltage 
could  then  be  raised  cautiously  without  further  disturbance. 
The  fact  that  the  gap  was  not  ruptured  showed  that  a  transient 
rise  of  voltage  occurred,  but  that  there  was  insufficient  energy 
to  cause  a  dynamic  arc. 

USE  OF  REACTANCE 

It  occurred  to  the  writers  that  the  generator  current  might  be 
considerably  reduced  by  using  a  shunt  reactance  to  neutralize 
the  leading  component  of  the  cable-charging  current,  thus  secur- 
ing a  better  wave  form  by  reducing  the  ampere  load  on  the  genera- 
tor. This  has  been  tried  abroad*  and  also  by  the  Edison  Electric 
Illuminating  Co.  of  Boston,  f 

We  made  nine  tests,  and  in  every  case  the  same  cable  was  used, 
namely  1000  ft.  (305  m.)  No.  1/0,  7/32  in.  (5.56  mm.)  wall,  rubber 
insulation,  having  a  capacity  of  0.175  microfarad.  Three  differ- 
ent tests  were  made  at  each  of  three  different  voltages.  At  each 
voltage;  first,  the  oscillogram  was  taken  without  the  reactance; 
second,  the  reactance  was  adjusted  until  the  line  current  was  a 
minimum;  and  third,  the  reactance  was  adjusted  for  the  best  volt- 
age wave  form.  The  results  of  these  tests  are  shown  in  Figs.  16A 
to  18c  inclusive. 

The  following  conclusions  are  to  be  drawn  from  the  above  tests. 

(1)  The  point  of  minimum  current  does  not  necessarily  corres- 
pond to  the  best  wave  shape. 

(2)  The  best  wave  shape  may   occur  at  an  abnormally  large 
value  of  lagging  current. 

(3)  The  wave  can  not  be  made  sinusoidal  in  every  case. 

(4)  At  the  point  of  minimum  current  (usually  denoting  re- 
sonance for  a  parallel  circuit)  the  power  factor  is  below  50  per- 
cent in  two  cases,  and  70  percent  in  the  third,  and  the  waves 
are  not  necessarily  in  phase. 

Further,  in  Figs.  ISA,  18B  and  18c,  when  the  generator  volt- 
age was  low,  there  was  but  slight  distortion  in  the  e.m.f .  wave, 
which  tends  to  confirm  the  previous  theories  as  to  the  effect  of  the 
transformer  exciting  current  on  the  e.m.f.  wave. 

Thus  with  a  commercial  generator  and  transformer,  the  e.m.f. 
wave,  by  the  use  of  reactance,  could  not  always  be  made  sinu- 
soidal, and  when  this  was  accomplished,  it  was  at  the  expense 
of  greater  generator  capacity.  This  is  undoubtedly  due  to  the 

*Electrotechnischer  Zeitschrift,  Feb.  27,  1908 

f" High- Potential  Cable  Testing  at  Boston,"  by  C.  L.  Kasson,  Electrical 
World,  Vol.  60,  p.  354. 


VOLTAGE    TESTING   OF   CABLES 


27 


FIG.  16  A 


FIG.  16  B 


FIG.  16  C 


28 


VOLTAGE  TESTING  OF  CABLES 


FIG.  17  A 


FIG.  17  B 


FIG.  17  C 


VOLTAGE    TESTING    OF    CABLES 


29 


FIG.  18  A 


FIG.  18  B 


FIG.  18  C 


30  VOLTAGE    TESTING   OF    CABLES 

fact  that  a  circuit  can  be  tuned  to  but  one  fequency  at  a  time, 
and  when  the  third  harmonic  current  was  neutralized,  the  react- 
ance allowed  a  very  arge  fundamental  to  pass.  The  power- 
factor  is  explained  by  the  fact  that  the  harmonic  currents  in 
cable  tests  predominate  in  the  current  wave,  whereas  the  voltage 
is  largely  fundamental.  The  harmonics  in  the  current  wave 
contribute  no  power  with  respect  to  the  fundamental  voltage, 
yet  all  add  up  in  quadrature,  contributing  to  the  volt-amperes. 
These  results  show  that  in  our  particular  case,  at  least,  a  shunt 
reactance  would  be  of  but  very  little  value  in  improving  the  gener- 
ator wave  form,  and  reducing  the  volt-ampere  load  of  the  genera- 
tor. 

SINE-WAVE  GENERATOR 

As  a  result  of  our  tests,  the  wire  manufacturer  came  to  the  con- 
clusion that  a  generator  of  the  ordinary  design  was  wholly  un- 
suited  for  reliable  testing  of  wires  and  cables.  Moreover,  no 
manufacturer  would  guarantee  a  generator  to  produce  an  approxi- 
mate sine  wave  under  these  severe  conditions  of  test.  The  ser- 
vices of  Prof.  C.  A.  Adams  of  Harvard  University  were  secured, 
and  under  his  specifications  such  a  generator  was  built  and  in- 
stalled, rated  at  85  kv-a.  All  oscillograph  records,  taken  up 
to  the  present  time  and  under  various  conditions  of  test,  have 
failed  to  show  any  departure  from  a  sine  wave. 

The  50-kv-a.  transformer  has  been  replaced  by  one  rated  at 
75  kv-a.,  75, 000  volts,  operating  at  a  much  lower  core-density 
than  the  former,  and  taking  a  much  less  distorted  exciting  cur- 
rent. Hence  the  distorting  influence  of  this  current  on  the  genera- 
tor wave  is  much  less  than  it  was  in  the  case  of  the  50-kv-a. 
transformer. 

METHOD  OF  MEASURING  VOLTAGE 

It  is  essential  to  obtain  reliable  knowledge  of  the  maximum 
voltage  to  which  the  cable  may  be  stressed  under  the  preceding 
conditions  of  distorted  wave  form,  if  the  tests  are  to  be  of  any 
great  value.  Where  the  wave  varies  from  a  peaked  to  a  flat- 
topped  wave,  the  effective  value  is  only  a  poor  indication  of  what 
the  maximum  voltage  may  be.  The  circuit  conditions  are  a 
function  of  so  many  variables  that  only  a  wide  experience  with 
his  apparatus  enables  an  operator  to  know  what  wave  form  may 
be  expected  under  any  given  set  of  conditions. 

The  spark  gap  immediately  suggests  itself,  as  a  means  of 
determining  these  peak  values.  Although  the  needle  gap  is  not 


VOLTAGE    TESTING   OF   CABLES  31 

conceded  to  be  a  device  of  high  accuracy  yet  it  is  accurate 
enough  for  the  work  in  hand.  There  are  objections  to  its  use, 
however.  The  voltage  can  only  be  determined  by  connecting  the 
gap  in  parallel  with  the  cable  to  be  tested,  and  noting  the  trans- 
former primary  voltage  when  the  gap  breaks  down  at  the  prede- 
termined value  at  which  it  is  set.  This  is  a  very  dangerous  prac- 
tise, as  a  disturbance  is  created  in  the  highly  oscillatory  circuit  al- 
ready described,  and  cables  have  often  been  known  to  puncture  at 
a  voltage  apparently  much  less  than  their  rating,  and  after  the 
gap  had  actually  broken  down.  The  spark  gap  can  therefore  be 
used  only  with  considerable  care,  and  the  danger  of  a  surge  is 
always  present.  Furthermore,  it  is  not  a  piece  of  apparatus  that 
is  easily  or  quickly  manipulated,  and  is  wholly  unsuited  for  a 
testing  room  where  a  large  number  of  tests  must  be  completed 
in  a  short  time. 

The  oscillograph  in  its  ordinary  form  is  a  very  satisfactory 
piece  of  apparatus  for  experimental  work,  but  it  requires  consider- 
able attention,  is  clumsy  to  handle,  requires  skill  to  manipulate, 
and  it  does  not  hold  its  calibration  for  any  considerable  time. 

Prof.  F.  A.  Laws  of  the  Massachusetts  Institute  of  Technology, 
and  the  writers,  have,  however,  adapted  the  oscillograph  principle 
to  an  instrument  which  may  be  placed  directly  on  the  switch- 
board, and  from  which  the  peak  value  of  any  voltage  wave  may  be 
quickly  and  accurately  determined.  A  sectional  view  of  this 
instrument  is  shown  in  Fig.  19,  and  is  almost  self-explanatory. 

The  lamp,  having  a  straight  tungsten  filament,  is  mounted  so 
that  its  distance  from  the  vibrator  may  be  adjusted  to  suit  the 
optical  requirements  of  the  system.  The  light  then  passes  through 
suitable  spherical  lenses  to  the  vibrator,  from  which  it  is  reflected 
through  a  cylindrical  lens  to  the  ground  glass  screen,  where 
the  peak  of  the  voltage  wave  may  be  determined  from  the  ex- 
tremity of  the  band  of  light. 

The  necessary  vertical  and  horizontal  adjustment  of  the  beam 
of  light  can  be  made  from  the  front  of  the  switchboard  by  means 
of  two  milled  heads  which  actuate  the  two  adjusting  rods.  To 
compensate  for  changes  in  the  amplitude  of  vibration  due  to 
variations  of  temperature  and  other  causes,  an  adjustable 
rheostat  is  connected  in  series  with  the  vibrator,  and  by  throwing 
the  vibrator  circuit  on  direct  current,  with  a  double-throw  switch, 
the  calibration  can  be  quickly  and  accurately  made.  A  double 
scale  is  also  provided.  The  magnets  are  operated  at  high  satura- 
tion so  that  fluctuations  in  exciting  current  affect  the  instrument 


32 


VOLTAGE    TESTING   OF    CABLES 


but  slightly.  The  instrument  as  used,  is  connected  to  the  sec- 
ondary of  a  potential  transformer,  whose  primary  is  connected 
directly  to  the  high-tension  circuit.  Other  views  of  the  instru- 
ment are  shown  in  Figs.  20  and  21. 

To  the  manufacturer  the  importance  of  this  type  of  instru- 
ment is  evident.  He  is  aware  of  the  maximum  stress  at  which 
his  cables  are  being  tested  at  all  times,  regardless  of  generator 
and  transformer  wave-form.  No  additional  factor  of  safety  is 


Pole  Piece 


Exciting  Co/fs 


Fixed  Resistance 

FIG.  19 — SECTIONAL  VIEW  OF  SIMPLEX  VIBRATING  VOLTMETER 


necessary  in  the  cable  due  to  uncertainty  on  this  point.  Further- 
more, purchasers  and  inspectors  can  be  quickly  and  convinc- 
ingly shown  that  their  cables  are  being  tested  at  the  specified 
voltage,  without  employing  a  troublesome  oscillograph  or  a 
spark-gap,  and  without  exposing  the  cable  to  the  dangers  accom- 
panying the  use  of  this  latter  device. 

This  instrument  is  not  only  useful  for  cable-testing,  but  can 
be  employed  to  advantage  where  apparatus  other  than  cables 
must  undergo  potential  tests.  The  e.m.f.  waves  of  all  testing- 
generators  are  not  sinusoidal,  even  at  light  loads,  and  their 
wave-form  may  change  with  the  field  excitation. 

When  the  e.m.f.  is  taken  directly  from  a  commercial  circuit 
supplying  other  loads,  this  voltage  wave  may  vary  with  the  load 
and  the  number  of  generators  on  the  system,  as  well  as  through 
the  compensators  and  other  control  devices  employed. 


VOLTAGE    TESTING   OF    CABLES 


33 


FIG.  20 


FIG.  21 


34  VOLTAGE    TESTING   OF   CABLES 

This  voltmeter  is  also  capable  of  indicating  slow-period  tran- 
sients which  the  ordinary  type  of  meter  owing  to  its  inertia  can- 
not follow.  Instances  of  this  have  come  to  the  writers'  attention 
in  the  cases  already  cited,  when  the  voltage  was  being  raised  on 
a  cable.  In  a  certain  power  system,  it  was  found  necessary  to 
change  the  lightning  arresters  from  2300  to  3300  volts  owing 
to  the  fact  that  continual  discharges  were  taking  place  due  to  a 
considerable  length  of  submarine  cable  having  been  added  to  the 
system.  Whether  this  was  due  to  a  change  in  wave-form  or  to 
surges,  the  writers  are  not  prepared  to  say,  but  such  an  instru- 
ment would  have  quickly  given  the  required  information. 

In  closing,  the  writers  wish  to  express  their  thanks  to  Prof. 
F.  A.  Laws,  of  the  Massachusetts  Institute  of  Technology,  for 
his  part  in  the  development  of  this  instrument;  to  Mr.  W.  G. 
Wolfe,  of  Boston,  for  his  assistance  in  developing  the  optical 
system;  to  Professors  C.  A.  Adams,  H.  E.  Clifford,  and  A.  E. 
Kennelly  of  Harvard  University,  for  their  helpful  suggestions 
and  criticisms  during  the  preparation  of  this  paper. 


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